Who were William Scrivener's parents?

William Scrivener was my 3xgreat-grandfather (my father's mother's great-grandfather). He appears with my great-great grandmother Harriet Scrivener or other members of the family in the censuses, where the following information is recorded:

• 1841: born around 1816, Bedfordshire (shown with wife and elder daughter)
  
• 1851: born around 1813, Limbury, Bedfordshire (shown with wife and daughters)
• 1871: born around 1815, Luton, Bedfordshire (shown with wife and daughters including Harriet)
  
• 1881: born around 1810, Limbury, Bedfordshire (shown with younger daughter Sarah)

I haven't yet found the family on the 1861 census, and I am still uncertain about identifying William in the 1891 census (he may be in the workhouse ). So now I want to try to track down Wiliam's birth/baptism, and the names (and marriage) of his parents, in order to step one generation back.

What else have I found out?

• Harriet Scrivener's birth certificate of 1851 names her parents as William Scrivener and Hannah formerly Hawkes
• William appears to have married Hannah Hawkes at St Mary Luton in February 1836; no parents' names are given in the parish register, although the witnesses were James Hawkes and Elizabeth Scrivener (James was Hannah's brother).
  
• The IGI has some possible baptism records:
  • William Scrivner, 19 May 1813, Stanbridge, Leighton Buzzard, Beds (parents John and Sarah)
  • William Scrivener, 6 August 1815, St Mary Luton, Beds (parents John and Sarah)
    
• The IGI also has some possible marriage records for John and Sarah Scrivener:
  • Jn Scribner to Sara Thest, Leighton Buzzard, Beds, 4 January 1800
  • Jn Scribner to Sara Freeman, Luton, Beds, 17 October 1807
• The IGI also has baptism records for other children (possibly) of both couples:
  • Thomas Scribner, 11 May 1800, Stanbridge
  • John Scribner, 27 May 1804, Stanbridge
  • Maria Scribner, 24 July 1808, Stanbridge
  • Bingman (?)  Scrivner, 1 August 1813, Stanbridge
  • ----
  • Harriet Scribner, 31 March 1809, St Mary Luton
  • Peter Scribner, 24 November, 1811, St Mary Luton
  • Miram Scrivner, 5 August 1827, St Mary Luton

I have tried to narrow the field by searching for these various Scribner/Scrivner/Scriveners in the 1841-1861 censuses. Sarah Thest appears under her married name, aged 65, living with her son William and his family in Stanbridge, whilst the Stanbridge Thomas is living there with his wife Catherine and family, so I have discounted the Stanbridge branch. 

As far as I can make out, 'my' William Scrivener's parents were John (or Jonathan) and Sara Freeman, both of whom appear to have died before the 1841 census, and whose only other known children were Miram (or Miriam), Harriet and Peter. I have not yet found any likely death records for them in the NBI.

So not much further forward (or backward) really!